The present standard method of countersinking large diameter holes in aircraft manufacturing operations is a highly regulated operation. Very close tolerances are held with countersink fastener hole diameters, surface perpendicularity, surface flushness, and surface angularity to hole. Therefore it is necessary to maintain a proper attitude of a countersink with respect to all of these measurements and tolerances. Repairing a mis-aligned countersink requires a larger size hole, larger countersink and a larger fastener to be installed. This adds time, cost, and weight to the manufactured product. The tolerances for workpiece surface flushness of 100 degree countersink fasteners may be -0.005 to +0.002 inches. Pre-drilled hole angularity must be held within 2 degrees because of aerodynamic flushness considerations.
U.S. patent application Ser. No. 08/788,727 is entitled "Blind Hole Self-Collet Countersink" and is hereby incorporated by reference in its entirety into this patent application. For convenience, this patent application Ser. No. 08/788,727 will hereinafter be referred to as the '727 system. L. D. Rissler is the sole inventor of the '727 system. Rissler is also the sole inventor of the invention described in this patent application. The '727 system attaches to a pre-drilled hole and countersinks perpendicular to the front side material or workpiece surface. The '727 system is fully automatic, but it lacks an improved externally indexing countersink pilot guide apparatus, hereinafter "external pilot guide", of this invention and is therefore dependent on a conventional countersink grind configuration to center the countersink cutter. Conventional countersink operations use an internal countersink pilot, which indexes to the center bore of the countersink. The internal countersink pilot allows no means for a hole large enough to accommodate the flare tube 14 '727. The unique external pilot guide of this invention substantially improves the performance of the '727 system and other systems of countersinking, counterboring, or spotfacing.